U.S. Pat. No. 4,189,675 proposes a satellite communications method and apparatus for communicating with mobile users using a satellite in a predetermined orbit.
EP 0562374 and EP 0568778 are believed to describe the "Iridium" proposed satellite cellular mobile communication system.
An alternative proposed satellite cellular system is described in EP 0536921 and EP 0506255.
GB-A-2295296 and WO-A-96/16488 describe a satellite communications network and in particular the ground segment thereof.
Communications to and from moving platforms such as aircraft or ships have traditionally been either ground based or satellite based.
One description of a satellite based system is given in "An airline passenger telephone system design development and early trials"; J. G. Schoenenberger and R. A. McKinlay, Fourth International Conference on Satellite Systems for Mobile Communications and Navigation, Oct. 17-19, 1988, pages 97-101, published by IEE.
In the system discussed, an aircraft carries equipment to allow two voice channels to carry passenger telephone conversations via a satellite to one or a number of ground stations. Four telephone handsets are provided in the aeroplane, and are assignable to the two channels.
U.S. Pat. Nos. 5,408,515 and 5,438,610 describe a satellite based system in which aircraft are equipped with a number of phones for use by customers. The customers register their seat numbers on check in, so that it is known which customer sits at which seat. Ground stations along the likely route to be taken by the aircraft are paged by the network on receipt of an incoming call by a party who knows on which aircraft and in which seat the customer is sitting, and when the page is successfully answered by the aeroplane, a call back is set up.
Dedicated ground to air systems are known which do not involve satellites, generally via dedicated HF ground stations. These systems, however, only give partial coverage over land and little coverage at sea.
U.S. Pat. No 4,399,330 discloses a communications system for a train, in which the train carries a number of telephones (one per compartment) and a mobile terminal concentrator device which communicates with the subscriber terminals and with a ground based radio connection station. The subscribers insert "smart cards" carrying ID codes or other data, and data indicating their presence or absence on the train is transmitted to the radio station.
Various terrestrial digital cellular communications systems are known or proposed. Of these, the GSM system is widely known.
In the GSM system, as in other cellular systems, in order to be able to direct incoming calls to a user, it is necessary to be aware of the location of the user (so called "mobility management"). In GSM, this is achieved by the provision of two layers of databases; so called "home location registers" (HLRs), and so called "visiting location registers" (VLRs), and by registration and location updating signalling processes.
Subscriber data on a given user and/or user terminal is stored in a specific HLR for that user. Each mobile switching centre (MSC) associated with a particular geographical area has an associated VLR, in which are temporarily stored details necessary for call management of all users currently thought to be within the area of that MSC.
Initially, a mobile terminal scans the broadcast common control channels (BCCHs) originating from all base stations (BSCs) within its reception, and attempts to register with one. The registration takes the form of an exchange of validation data, as described in "Security aspects and the implementation in the GSM-system", Peter C. J. van der Arend, page 4a, Digital Cellular Radio Conference (DCRC) Conference Proceedings, Oct. 12-14, 1998, published by Deutche Bundespost, France Telecom and FernUniversitate.
If the data match, the mobile terminal is registered as being within the area of the VLR and MSC. On registration, the identity of the VLR is stored in the HLR for the mobile terminal.
Subsequently, when incoming calls arrive for the mobile terminal, the HLR is accessed to determine the VLR where the mobile is registered and calls are then routed to the MSC associated with that VLR.
Even in idle or "sleep" mode, mobile terminals will continually or periodically scan the broadcast control channels (BCCHs) which they can receive. Each BCCH of a cell carries, amongst other data, a location area identifier (LAI) indicating an individual cell or a group of cells within a certain area. When a new location area indicator is detected (indicating that the mobile terminal has moved into a new area), the mobile terminal transmits a location update request indicating the new LAI.
The exchange of authentication data is repeated, and if the mobile terminal is authenticated the new location area indicator is written into the VLR.
Thus, the VLR continually maintains an indication of which area (and, more specifically, which cell) the mobile terminal is within.
A mobile terminal may also move between the areas of two different VLRs.
In the GSM system, there is also a signalling procedure to secure periodic registration from mobile stations, to maintain information on the status of mobile stations.
Similar issues will arise in relation to satellite communications systems; see, for example, the paper "Study on network issues of medium earth orbit satellite communications systems"; Araki et al, Proceedings of the Third International Mobile Satellite Conference IMSC 1993 (JPL publication 93-009),pages 529-534, published by Jet Propulsion Laboratories (1993). In that paper it is described how either each land earth station may issue a location area identifier signal which is carried by spot beams of satellites within the area, or each spot beam of each satellite may carry a location area identifier.
Various attempts have previously been made to integrate cellular telephony and calls to aircraft. Naturally, mobile telephone users wish to use their telephones on board moving platforms such as a aircraft, but this leads to a number of problems.
Firstly, there may be interference with on board navigation systems or other equipment.
Secondly, there may be interference between aircraft systems and other systems through the coverage regions of which the aircraft is flying.
Thirdly, due to the relatively high speed with which the customer may be moving, terrestrial systems may be insufficient to cope with the number of handovers and the sudden transient loads as the moving platform passes through their coverage region.
U.S. Pat. No 5,444,762 discloses a system in which an aircraft monitors terrestrial channels and makes use of unused channels.
One approach is to provide a local cellular base station and switching centre on board the vehicle, to treat the vehicle as a microcell. Thus, the aircraft itself can deal with all signalling to and from the cellular telephones to handle location updating.
For example, "The provision of GSM cellular radio environments within passenger aircraft operating over Europe", I. Casewell, Fifth International Conference on Mobile, Radio and Personal Communications (Dec. 11-14, 1989) pages 57-67 published by IEE, discloses a system in which an aircraft has a transponder which includes a first air interface for communicating with mobile telephones on board, and a second air interface for communicating with terrestrial base stations.
WO 94/28684 suggests providing an aircraft with a GSM station including both a base station and a mobile switching centre, linked to terrestrial networks by a dedicated HF network. The user terminals in this case are conventional cellular handsets coupled via their external antenna sockets to connecting wires.
AU 9477530 suggests a dual mode handset which can either operate as a traditional RF handset, or through an infrared optical link when on board a vehicle such as a plane, which has a base station with an infrared interface.
Several of the above described systems are unsatisfactory in various ways. Those systems which make use of RF communications within the vehicle are not preferred because of the possibility of interference with vehicle electronic systems. On the other hand, separate systems providing non-cellular communications facilities inevitably require separate payment, and make it difficult or impossible to receive incoming calls.
Finally, the provision of base station and mobile switching centre equipment on board a vehicle for what may be a relatively small number of users is unnecessarily expensive.
The present invention, in various embodiments, is directed to alleviating one or more of the above problems.
According to one aspect, the present invention provides a multi user communications terminal apparatus providing simultaneous access for a plurality of users to a communications network in which the geographical locations of each user are registered and periodically updated, the terminal apparatus comprising a plurality of user terminals each including electroacoustic transducers; and a common RF unit comprising a shared RF amplifier for coupling to a shared antenna system, in which the terminal apparatus comprises means for transmitting common location updating signals, and is arranged not to transmit separate location update signals for every one of said users.
In another aspect, the invention provides a method of communication from a multi user terminal comprising the step of sending common location updating messages to jointly update the positions of a plurality of users of said terminal.
In another aspect, the present invention provides a method of mobility management in a mobile communications system for a plurality of mobile users, comprising updating position data for a plurality of mobile users of a multi user mobile terminal on the basis of common updating messages for said users.
In another aspect, the invention provides mobility management apparatus in a mobile communications network, for registering and periodically updating geographical locations of a plurality of users of said network, the apparatus being arranged to update the geographical locations of a plurality of users of a multi user communications terminal apparatus in dependence upon a smaller number of location updating messages received from the multi user communications terminal apparatus.
In these aspects, a considerable reduction in the volume of signalling traffic is achieved, since it is not necessary continually to update the positions of all users on board the terminal; it is preferably not even necessary to update the positions of as many users as can be allowed simultaneously to communicate via the terminal. Preferably, only a single location updating message for all the users is transmitted.
At the same time, location updating can be performed more accurately since the terminal can have access to sophisticated positioning systems such as GPS which may be present in the vehicle.
In another aspect, the present invention provides a multi user communications terminal apparatus providing simultaneous access for a plurality of users to a communications network in which the geographical locations of each user are registered and periodically updated, the terminal comprising: a plurality of user interface units each including electroacoustic transducers and each lacking an RF stage capable of direct communication to said communications network; a plurality of signal processing units each capable of being associated with one of said user interface units; and a common RF unit to which each of said signal processing units is connectable.
By using a common RF stage and not allowing the users individual RF interfaces, interference with on board electronics and/or surrounding telecommunications systems is minimised, and the cost of the user terminals is reduced.
On the other hand, by providing a plurality of signal processing units (e.g. including the baseband processing stages such as low bit rate coding, signalling and encryption) each communicating user terminal can function almost exactly as a conventional cellular handset, allowing the multi user terminal apparatus readily to be integrated into a satellite or terrestrial cellular network.
By maintaining separate signal processing circuits (e.g. each including a signalling stage) for multiple users, routing of incoming calls to different users is facilitated.
In one embodiment, a signal processing circuit is provided in each user terminal; thus each user terminal functions almost exactly as a conventional cellular handset, but without an individual RF stage (and preferably, as discussed above, without separate position updating messages).
The maximum power available through the common RF stage may be limited to less than the power required to simultaneously support all user terminals. In this case, each user terminal is preferably arranged to enter into a signalling dialogue with the RF stage to determine whether sufficient power is available for it to communicate, prior to setting up a call.
In another embodiment, fewer signal processing circuits than the total number of user terminals are provided, and signal processing circuits are allocated to users as needed via a routing circuit.
This further reduces hardware cost, since the number of signal processing circuits may be constrained to match the total power available from the RF stage.
Preferably, in this case, a store stores details of the correspondence between different users and their respective user terminals, to assist in allocating an incoming call to the right user.
Other aspects and preferred embodiments of the invention are as described or claimed hereafter, with advantages which will be apparent from the following.